Gene diversification is thought to occur largely by duplication followed by functional divergence. Näsvall et al. tested whether functional changes might also precede duplication and propose an innovation–amplification–divergence (IAD) model. HisA and TrpF are bacterial enzymes used for the synthesis of histidine and tryptophan, respectively. The authors studied the evolution of multiple lineages of trpF-deficient Salmonella enterica under amino-acid-limiting conditions. They found that hisA evolved dual functionality to act in both the histidine and tryptophan biosynthetic pathways, followed by gene amplification and specialization of the gene copies. Thus, IAD may underlie some evolutionary processes.